Several systems have been proposed to record and/or reproduce television video signals on a magnetic tape. One such system is the helical-scan video tape recorder (VTR) wherein a tape wrapped around a drum, which includes a rotary head, is transported to form video tracks or recorded paths of a video signal on the tape diagonally with respect to the longitudinal direction of the tape. Such a helical-scan VTR is widely used by consumers as well as in industry and by professionals in broadcasting stations. In the helical VTR, the video signal of one or more fields is usually recorded on a single video track, and the rotary head is rotated in synchronism with a vertical synchronizing signal for the input video signal to be recorded.
In the following description, each video track is assumed to contain one field of the television video signal. It is apparent, however, that the scope of the instant invention is not limited to this particular case.
In the case where the video signal of one field is recorded on one video track, the rotation of the rotary head is controlled so that the vertical blanking portion of the television video signal is recorded at end portions of each video track (the margin of the tape). In a single-head helical VTR, wherein the television video signal is recorded by a single rotary head, the rotation of the single rotary head is controlled so as to rotate one revolution in one field period. In a dual-head helical VTR, wherein the television video signal is recorded by two rotary heads disposed apart from each other by 180.degree., the rotation of the rotary heads is controlled so as to rotate one-half revolution in one field period. Such control is accomplished by phase comparing a tachometer signal representing the rotating phase of the rotary head and a vertical synchronizing signal from the television video signal to be recorded. Accordingly, the vertical synchronizing signal for the television video signal to be recorded maintains a predetermined phase relationship with the tachometer signal in the record mode.
For reproducing the recorded video signal, the rotary head is controlled so as to rotate at a predetermined rotational speed in synchronism with a synchronizing signal being supplied from a reference sync generator or other reference signal. In normal reproduction, wherein the tape is transported at normal tape speed equal to the recording tape speed at which it was recorded, the running phase of the tape is controlled by controlling the rotation of the tape driving capstan so that the rotary head follows the center of a video track (achieves "tracking"), i.e., the head scanning path on the tape coincides with the video track.
The television video signal reproduced from the VTR contains a time-base fluctuation due to mechanical fluctuation in the rotation of the rotary head and the tape transport speed. Such time-base fluctuation can be corrected by a digital TBC. In a conventional digital TBC, as described in U.S. Pat. No. 3,909,839 entitled "PHASE LOCKING SYSTEM FOR TELEVISION SIGNALS USING DIGITAL MEMORY TECHNIQUES" issued to Inaba et al. and assigned to the present assignee, and in copending U.S. patent application Ser. No. 941,936 entitled "DIGITAL TIME-BASE CORRECTOR" by Mr. Mizukami, now U.S. Pat. No. 4,214,262, assignor to the present assignee, and one of the present joint inventors, a VTR-reproduced signal containing a time-base fluctuation is converted to a digital television video signal. The conversion is in response to a write-in clock pulse synchronized with a sync signal contained in the reproduced signal and the digital signal is stored in a digital memory. The stored digital television video signal is read out in response to a read-out clock pulse synchronized with a reference sync signal having no time-base fluctuation, and is converted to an analog television video signal. Thus, a television video signal containing a time-base fluctuation is stored in response to a write-in clock pulse containing the fluctuation and is read out in response to a read-out clock pulse containing no fluctuation, with the result being that no time-base fluctuation is contained in the read-out television video signal.
Because the helical-scan VTR records the television video signal of one field on a single video track, the television signals recorded on two adjacent video tracks have a very high correlation. Therefore, as described in the U.S. Pat. No. Re.27,371, now abandoned, entitled "RECORDING SYSTEM WITH PROVISION FOR FAST OR SLOW REPRODUCTION" issued to S. Okamura, the helical-scan VTR can produce a special-motion television video signal having a motion proportional to the reproducing tape speed when the tape is transported at a reproducing tape speed different from the normal tape speed and when the rotation of the rotary head is maintained at the recording rotational speed. In other words, special-motion reproduced television video signals, such as fast-, slow- and still-motion television video signals, can be produced when the tape is driven at a higher than normal reproducing tape speed, at a lower than the normal reproducing tape speed, or is stopped, respectively. In such special reproductions, the rotary head scans more than one video in one scanning period. This is in contrast to the rotary head scan following the center of the video track in normal reproduction.
The signal pattern on a tape recorded with a helical scan VTR is normally formed so that the corresponding horizontal synchronizing signals on two adjacent video tracks have a certain spacing provided between them. Therefore, when a rotary head scans across more than one video track, in the period of one field, the number of horizontal scanning lines reproduced in the period of one field will change by a degree proportional to the spacing between adjacent video tracks, and the number of video tracks across which the head is scanning. A change in the number of horizontal scanning lines in the period of one field means a change in the horizontal scanning period, or, in other words, a time-base change is contained in the reproduced television video signal. Consequently, the television video signal reproduced in a special reproduction mode contains a time-base error which comprises both a time-base fluctuation, inherent in the VTR reproduced signal, and a time-base change due to the special reproduction mode. The amount of the time-base change increases with the increase in the tape transport speed.
Copending U.S. patent application Ser. No. 91,195, entitled "VIDEO HEAD DEFLECTION APPARATUS FOR SPECIAL MOTION REPRODUCTION BY HELICAL SCAN VTR" by Y. Sato and T. Konishi, assignors to the present assignee, of whom T. Konishi is one of the present joint inventors, proposes an improved special reproduction system wherein the rotary head is deflected in a direction vertical to the video track so that the head will follow a single video track during a single scanning even when the tape is transported at a speed different from normal speed. In this improved special reproduction system, before the rotary head completes a single scanning of a single video track, it is displaced to the position of another video track, thus causing a change in the number of horizontal scanning lines being reproduced. Consequently, even the television video signal reproduced by such an improved reproduction system contains not only a time-base fluctuation inherent in the VTR reproduced signal but also a time-base change due to the special reproduction mode.
In the conventional TBC, a color burst signal is extracted from the horizontal sync signal for a reproduced television video signal, and the extracted color burst signal is applied to a burst controlled oscillator (BCO), whereupon a write-in clock pulse in synchronism with the reproduced television signal is generated. Therefore, when a television video signal, reproduced in the special reproduction mode contains a time-base change, i.e., there is a great change in the horizontal synchronizing frequency, it is very difficult for the conventional TBC to extract a color burst signal.
When a color burst signal can be extracted from a reproduced television video signal, containing a time-base change as well as time-base fluctuation, and a write-in clock pulse in synchronism with the reproduced television video signal can be produced in response to the extracted color burst signal, the conventional TBC can correct the time-base fluctuation but it cannot correct the time-base change, i.e., a change in the number of horizontal scanning lines.
Therefore, one object of the instant invention is to provide a digital TBC having a color burst separator circuit capable of extracting a color burst signal from a television video signal that has undergone a change in the horizontal synchronizing frequency.
It is another object of the instant invention to provide a digital TBC for correcting a television video signal, that has undergone a change in the number of horizontal scanning lines, to a television video signal having the normal number of horizontal scanning lines.
It is a further object of the instant invention to provide a digital TBC for correcting a time-base error in a television video signal which includes not only time-base fluctuation but also time-base change.